How to define the diameter to a certain size
Using curve objects in blender you can set the size of the extruded shell that follows the curve and also adjust it along the path as desired.

There are two ways to turn a curve into a tube. For a quick circular bevel, just set the bevel depth and resolution for the curve, you will also want the fill to be set to full. The other way is to use another curve object to define the outer shape of the bevel.



The bevel depth is radius in blender units of the extrusion. If you use a bevel object then the size of the object used will determine the size of the extrusion.

Each point on a curve has a Radius property. At 1.0 the extrusion will match the bevel size set at that point. Other values will reduce or increase the size, so a point having a radius of 0.25 will be 1/4 the size of the main "pipe".

You can find the radius property in the properties sidebar T under Transform while editing a curve. You can also use &#9095; AltS to change the size interactively. A linear falloff is used between each points radius.



So once you set the main size in the bevel size, to adjust the size of specific points you will need to do a little calculation which can be done while entering the value to use. For example if you have the main bevel set to 0.95 and want 0.75 at a given point, in the radius enter .75/.95 and it will calculate the radius of .789 that you want.